st: Using stata for event studies

I have to do an event study. So I am thinking about using Stata. But I
have a question about the expected returns before I start:
I set up an estimation period of 250 trading days. I use OLS to get my
alphas and betas. How many observations of the stock price do I need
for the regression. Would it be the best, if I have the stockprices
for every 250 trading days? I dont really know how many observations
would be the best for the regression.